Liquid crystal devices are of increasing interest for electrically controllable displays. The discovery of liquid crystals which operate at room temperatures has encouraged an ever-expanding market for liquid crystal cells for watch displays, calculator displays, various automotive dial displays and the like. Liquid crystal cells operate either in a dynamic scattering mode or in a field effect mode.
One type of field effect cells are those characterized as twisted nematic liquid crystal cells. These cells derive their name from the initial ordering of the liquid crystal material, which has a twist of 90.degree. in alignment of the molecules throughout the cells. These cells employ liquid crystals or mixtures of liquid crystals which have positive dielectric anisotropy, i.e., the magnitude of the dielectric constant in a direction parallel to the long axis of the molecular chain is greater that the magnitude of the dielectric constant in a direction perpendicular to the long axis of the molecular chain. When an electric field is applied to the cell, the molecules of liquid crystal reorient themselves, thereby changing the passage of light through the cell as viewed through crossed polarizers. When the electric field is removed, the liquid crystal material reverts to its original alignment. The amount of voltage required to initiate the "untwisting" of the material of the cell is called the threshold voltage, and is inversely related to the anisotropy of the composition.
Electro-optic devices made from these twisted nematic liquid crystal materials have several stringent requirements for use in watches they should have a use temperature range of from about -10.degree. to 60.degree. C. or broader; the response time should be 250 milliseconds or less; and the threshold voltage should be less than 2 volts, and preferably less than 1 volt. Generally, devices of this type are operated at 2.5 to 3 times the threshold voltage in order to obtain good visual contrast in the display. Since electronic watches generally operate on a 1.5 volt battery, and liquid crystal materials which have a threshold voltage of less than 1.0 are very rare, some means of doubling or even tripling the voltage of the battery must be provided. This of course adds to the expense of watch manufacture. This expense would be avoided if liquid crystal materials were available to meet all of the other requirements for good field effect twisted nematic liquid crystal cells and in addition have lower threshold voltage.